High-resolution, passively controlled liquid crystal display

ABSTRACT

With a view to improving the resolution of passively controlled liquid crystal displays, the invention proposes that at least the orientation layer (14.1, 14.2) of one of the two substrate plates (3,4) of the liquid crystal display should be made up of a highly fluorated and preferably perfluorated, aliphatic polymer, which could be, among others, Teflon AF 1600 marketed by the Dupont Company. 
     An orientation layer formed in accordance with the invention also makes it possible for organic dyestuffs to be used as the chromophore layer, because an orientation layer in accordance with the invention only requires heat treatment temperatures not exceeding 250° C. The pretilt angles obtained by means of the orientation layers in accordance with the invention can be set during the manufacturing process and lie in the range between 25° and about 90°.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns passively controlled liquid crystaldisplays wherein the orientation layer arranged on the substrate platesimpresses a large pretilt angle, as well as a method of producing suchliquid crystal displays.

2. Description of the Prior Art

Liquid crystal displays have been known for some time in the state ofthe art. They have the common feature of consisting of two transparentplates that run parallel to each other and at a given distance betweenthem, as well as of a frame that rigidly joins these plates, the spacedelimited by the frame and the two plates being filled with liquidcrystal. Especially in high-quality liquid crystal displays suitable foruse in television sets, the two plates will be made of glass. The platesurfaces facing each other are coated with a variety of differentmaterials.

Thus, in the case of monochromatic liquid crystal displays, where thetwo plate surfaces facing each other are provided with the same sequenceof coatings, it is known first to provide the said plate surfaces with atransparent and structured electrode array, possibly as an ITOarrangement. This electrode layer is followed by an equalization andinsulation layer, which may be a layer of silicon dioxide for example.The equalization and insulation layer carries the socalled orientationlayer. This orientation layer, which is in direct contact with theliquid crystal, is traditionally made of polyimides or polyphenyls. Witha view to improving the orientation of the liquid crystal moleculesimmediately adjacent to the orientation layer, the surface of theorientation layer facing the liquid crystal can also be oriented bymeans of mechanical processes.

When the liquid crystal display is to be capable of reproducing colors,one of the two substrate plates, preferably the substrate plate on whichthe reproduction of--say--the television image is subsequently to beobtained, will be provided with an additional layer between the surfaceof the glass plate facing the liquid crystal and the electrode layer.This layer, which contains the organic dyestuffs, will henceforth bereferred to as the chromophore layer.

High-resolution, passively controlled liquid crystal displays, whichhave a wide angle of view and have to be highly multiplexed, call fororientation layers that impress a large angle of incidence, the socalledpretilt angle, upon the liquid crystal molecules.

Orientation layers having the required properties can be produced bymeans of oblique vacuum deposition of silicon dioxide on the substrateplate. Very large pretilt angles can be set when this technique isemployed. A disadvantageous feature is however constituted by the factthat this method is very costly and, what is more, can be used only withvery small substrate plates. When larger substrate plates have to beproduced with a pretilt angle up to about 25°, it is known that organicorientation layers, possibly a polyphenyl layer, can be applied to theformation of a polyphenyl layer on a substrate plate calls for temperingconditions of the order of 400° C. and all known dyestuffs from which agiven chromophore layer could be formed will change their previouslystable color above a temperature of about 240° C. to 250° C., intensiveefforts were subsequently made--bearing in mind that the chromophorelayer is arranged between the surface of the glass plate and theelectrode layer and that it must therefore quite inevitably be heated toabout 400° C. when the orientation layer is formed--to lower thetempering temperatures to a value that would not damage the chromophorelayers. This requirement was satisfied by the development of thesocalled low-temperature polyimides and low-temperature polyphenyls,which only call for a tempering temperature of the order of 160° C. to260° C. Although the liquid crystal displays produced by employing thesesubstances are already characterized by good color reproductionproperties, further investigations have shown that orientation layersformed with low-temperature polyimides and/or low-temperaturepolyphenyls have pretilt angles of up to 8° at the very most. Attemptsto combine a substrate plate devoid of a chromophore layer but providedwith a high-tilt orientation layer tempered at a temperature of about400° C. with a substrate plate having an orientation layer made up oflow-temperature polyimides have shown that the total tilt that can beobtained between the two substrate plates rises above the individualtilt in the orientation layer constituted by the low-temperaturepolyimides. But even the total tilt of these combined substrate plates,which is not equal to the arithmetic mean of the individual tilts butmerely approximates the individual tilt of the orientation layer havingthe larger coupling energy, is not yet sufficient for high-resolutionliquid crystal displays. The present invention involves a liquid crystaldisplay, and more particularly an orientation layer, that can be easilyproduced and will impress a large pretilt angle upon the liquid crystalmolecules, as well as a method of producing an orientation layer thatwill guarantee a large pretilt angle.

SUMMARY OF THE INVENTION

As far as the device itself is concerned, this problem is solved byproviding at least one of the two substrate plates with an orientationlayer made up of a highly fluorated and preferably perfluorated polymer.

The provision on substrate plates of orientation layers in accordancewith the present invention is also particularly advantageous in that itleads to a considerable improvement of the degree of blackening of theliquid crystal display in the switched state. When a substrate plateprovided with a polyphenyl layer tempered at 400° C. as the orientationlayer is combined with a substrate plate in accordance with the presentinvention, the magnitude of the total tilt between the two substrateplates is decisively affected by the individual tilt of the substrateplate of traditional design. This is true quite independently of whetherthe individual tilt of the substrate plate of traditional design isgreater or smaller than the individual tilt of the substrate plate inaccordance with the present invention.

This is particularly advantageous because the combination of substrateplates of traditional design with substrate plates in accordance withthe present invention makes it possible to accurately set practicallyany desired total tilt.

It is believed this feature of the highly fluorated aliphatic polymer asused in the present invention arises from the fact that this material isdistinguished by an extremely low surface energy. Moreover, the lowsurface energy of this material is also directly connected with theenhanced degree of blackening of a liquid crystal display with anorientation layer in accordance with the present invention, because thelesser coupling to the surface already ensures a weak field and a fullyhomoeotopic orientation.

The present invention also provides a method of producing an orientationlayer made up of highly fluorated aliphatic polymers. The specialreference to the amorphous fluoropolymer material known as TEFLON AF1600 is only intended by way of example, and the method can be usedespecially with any soluble, highly fluorated aliphatic polymer.

When the amorphous fluoropolymer, TEFLON AF 1600 of the Dupont Companyis used as the highly fluorated aliphatic polymer, one enjoys theadvantage that this material calls only for heat treatment temperaturesup to about 250° C., even though it ensures an angle of incidence of theliquid crystal molecules with respect to the surface of this orientationlayer of more than 25°.

When one of the two substrate plates is also provided with a chromophorelayer and this substrate plate with the chromophore layer is thenprovided with an orientation layer in accordance with the presentinvention, this brings the advantage that, over and above the largepretilt angle on this orientation layer, it is also possible to useknown dyestuffs for forming the chromophore layer. This is due to thefact that the highly fluorated aliphatic polymer used for forming theorientation layer can make do with heat treatment temperatures lyingbelow 260° C., the precise temperature depending on the nature of thepolymer.

The steps of the present invention are such as to exert a decisiveinfluence on the magnitude of the pretilt angle during the manufacturingprocess for producing an orientation layer in accordance with thepresent invention. When the solution applied to the substrate plate isfirst thoroughly dried and then subjected to the heat treatment neededfor the production of an orientation layer in accordance with thepresent invention, the heat treatment temperature acting on thesubstrate plate can be used to very accurately set the pretilt angle ofthe orientation layer. Temperatures close to the 250° C. limit will leadto comparatively small pretilt angles, while lower temperatures willproduce pretilt angles up to 90°.

DESCRIPTION OF THE DRAWING

The invention will now be discussed in greater detail by reference to afigure. This figure shows a section through a liquid crystal display inside elevation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The space 1, which is filled with liquid crystal 2, is formed by a firstsubstrate plate 3, a second substrate plate 4 and a frame 5, thesubstrate plates 3, 4 and the said frame 5 being rigidly glued to eachother and in such a way as to exclude the entry of air.

These substrate plates 3, 4, which run parallel to each other at a givendistance between them, consist in each case of a base plate 6, 7 made ofglass that on the side facing the liquid crystal 2 carries a series ofcoatings placed on top of each other. The layer in direct contact withthe surface 8 of the glass plate 6 is the socalled chromophore layer 9.This layer is immediately followed by the electrode layer 10.1, whilethe electrode layer 10.2 of the substrate plate 4 is in direct contactwith the glass plate 7. In the example here illustrated/the twoelectrode layers 10.1, 10.2 are formed as structured ITO layers and eachof them is provided with a large number of separate conductor strips11.1, 11.2. The conductor strips 11.1, 11.2 of the electrode layers10.1, 10.2 are turned through 90° with respect to each other, theirarrangement being such that the conductors 11.1 of the electrode layer10.1 run along the direction in which the figure is viewed, while theconductor strips 11.2 of the electrode layer 10.2 run in the directionof the paper on which it is printed.

The surfaces of the electrode layers 10.1, 10.2 that face each other arein each case covered by an equalization and insulation layer 12.1, 12.2,the layer--as can be clearly seen in the case of the substrate plate3--equalizing also the interstices 13 between the conductor strips 11.1and 11.2 (though in the latter case this cannot actually be seen fromthe figure). In the illustrated embodiment, the equalization andinsulation layer 12.1, 12.2 is a layer of silicon dioxide.

The equalization and insulation layer 12.2 of the substrate plate 4 isfollowed by an orientation layer 14.2 of the traditional and knownpolyphenyl type. The pretilt angle that is formed between the surface 17of the orientation layer 14.2 and the liquid crystal 2 amounts to about20°.

In another embodiment that is not shown here, though it is similar tothe embodiment as discussed, the orientation layer 14.2 of the substrateplate 4, i.e. the plate without a chromophore layer, can also consist ofa highly fluorated, aliphatic polymer. The special properties of thismaterial as a material for the orientation layers of liquid crystalcells will be discussed in some detail at a later stage.

The equalization and insulation layer 12.1 of the substrate plate 3,which is the substrate plate provided with the chromophore layer 9, isalso covered by an orientation layer 14.1. The material making up theorientation layer 14.1 is a highly fluorated, aliphatic polymer and canbe obtained under the commercial designation TEFLON AF 1600 (anamorphous fluoropolymer) from the Dupont Co. According to data suppliedby the manufacturer, this transparent material has a vitrificationtemperature of 140° C. to 160° C. and a melting point of 240° C. to 275°C.

In the embodiment here described, the orientation layer 14.1 wasproduced by first dissolving the TEFLON AF 1600 in perfluoratedfluorocarbon. When this is done, the proportion of TEFLON AF 1600 in thesolution should not exceed five percent in order to maintain theviscosity within proper limits. Subsequently, the solution was appliedto the surface 16 of the equalization and insulation layer 12.1 and,after being allowed to act on the surface 16 for a period of 30 seconds,was subjected to centrifuging at 1000 r.p.m. for a further 30 seconds.Following thorough drying, the orientation layer 14.1 was then heattreated at 250° C. for a period of 30 minutes. The pretilt angle of theorientation layer produced in this manner amounts to about 35° in thetransition area between the surface 15 of the orientation layer 14.1 andthe liquid crystal 2, while--given the small surface energy of theorientation layer 14.1--the total tilt prevailing between theorientation layers 14.1 and 14.2 amounts to nearly 25°.

Attention is drawn to the fact that the pretilt angle of everyorientation layer made of a highly fluorated, aliphatic polymer (in theembodiment here described only the orientation layer 14.1) depends onthe concentration of the highly fluorated, aliphatic polymer and thesolvent. The val stated hereinabove relate to a 1% solution of TEFLON AF1600 in a solvent marketed by the 3M Company under the commercialdesignation Fluorinert FC 77.

The surfaces 15, 16 of the orientation layers 14.1, 14.2, i.e. thesurfaces facing the liquid crystal 2, are oriented by means oftraditional methods.

It should also be noted that orientation layers made of highlyfluorated, aliphatic polymers are not limited to liquid crystal displayssuitable for color reproduction. When monochromatic liquid crystaldisplays are to be provided with an orientation layer in accordance withthe present invention, a sufficiently large total tilt between thesubstrate plates may already be obtained when only one of the twosubstrate plates is provided with the orientation layer made up of ahighly fluorated, aliphatic polymer.

What is claimed is:
 1. A high-resolution, passively controlled liquidcrystal display comprising:a first substrate plate (3) made of glass andcoated on one side with a transparent and structured electrode (10.1),an equalization and insulation layer (12.1) and a mechanically orientedorientation layer (14.1); a second substrate plate (4) made of glass andcoated on one side with a transparent and structured electrode (10.2),an equalization and insulation layer (12.2) and a mechanically orientedorientation layer (14.2); and a frame (5) joining the two substrateplates (3,4) along their coated surfaces in such a way as to prevent theentry of air; and a liquid crystal (2) contained in the space (1) formedby the two substrate plates (3,4) and the frame (5),wherein theorientation layer (14.1, 14.2) of at least one of the two substrateplates (3,4) is made of a transparent, highly fluorated, solubleamorphous, aliphatic polymer.
 2. A liquid crystal display in accordancewith claim 1, wherein the highly fluorated, soluble, amorphous,aliphatic polymer is a material marketed by the DuPont Company under thecommercial designation TEFLON AF
 1600. 3. A liquid crystal display inaccordance with claim 2, wherein one of the substrate plates (3,4)comprises a chromophore layer (9) between the surface (8) of the glassplate (6) and the electrode layer (10.1), the chromophore layer beingmade up of organic materials, and the substrate plate (3) provided withthe chromophore layer (9) is always the substrate plate that is providedwith an orientation layer (14.1) that is made up of the highlyfluorated, soluble, amorphous, aliphatic polymer.
 4. A liquid crystaldisplay in accordance with claim 1, wherein one of the substrate plates(3,4) comprises a chromophore layer (9) between the surface (8) of theglass plate (6) and the electrode layer (10.1), the chromophore layerbeing made up of organic materials, and the substrate plate (3) providedwith the chromophore layer (9) is always the substrate plate that isprovided with an orientation layer (14.1) that is made up of the highlyfluorated, soluble, amorphous, aliphatic polymer.
 5. A liquid crystaldisplay in accordance with claim 1, wherein the highly fluorated,soluble, amorphous, aliphatic polymer is a perfluorated aliphaticpolymer.